Our research focuses on the identification and characterization of genes critical for structure and function of sensory hair cells in the cochlea and vestibule. Spontaneous mutations that cause balance dysfunction and hearing loss are ideal tools to identify genes important for the functioning of the inner ear and to elucidate their role in these sensory systems. Towards this end we concentrate our efforts on three deafness mutations: jackson circler (jc) and Varitint-waddler (Va). By auditory-evoked brain stem response analyses we showed that in these strains hearing impairment is completely penetrant and fully expressed in three to four week old animals. The vestibular phenotype is also fully expressed in je and Va mutants, but seems to vary in jc. To search for hearing and balance modifiers we outcrossed each of the mutations onto different genetic backgrounds. To identify their molecular identity we analyzed large segregating intersubspecific intercrosses. We constructed physical BAC contigs for each of the mutated loci and we evaluated candidate genes. The molecular cloning of these mutations should give us new insights in the development and function of the mammalian cochlea. High-resulution genetic and physical mapping of varitint-waddler identified two new members of the mucolipin gene familiy Mcoln2 and Mcoln3 in the critical interval. We found missense mutations in Mcoln3 which are responsible for deafness and pigmentaton defects in Va. Mcoln3 shows sequence and motif similarities to the transient-receptor-potential (TRP) family of ion channels. Mcoln3 localizes to cytoplasmic compartments and stereocilia of outer and inner hair cells in the organ of Corti. Based upon its motif structure and expression domain, we suggest that Mcoln3 is involved in the regulation of ion homeostasis in hair cells and melanocyte. These studies identified a new molecular link between deafness and pigmentation defects. Finally, Mcoln2 and Mcoln3 are candidate genes for hereditary and/or sporadic forms of neurosensory disorders. To gain insights into the function of Mcoln3 we refined the stereociliary localization of Mcoln3 and investigated cochlear hair cell function in varitint-waddler mutant mice. Using the Mcoln3-specific HL4460 antibody we found a 64 kDa protein with equal expression levels in cochlea and vestibule in wildtype and VaJ mutants. By immunofluoresence on whole-mount organ of Corti preparations, we detected Mcoln3 in abundance near the base of developing stereocilia, coinciding with the position of ankle links. Mcoln3 localization is tightly regulated during the postnatal period and no longer detectable at postnatal day 10. Electrophyiological recordings in collaboration with Dr. Corne Kros and Dr. Guy Richardson, University of Sussex, revealed reduced mechano-electrical transducer currents in hair cells isolated from VaJ/+ and VaJ/VaJ mice. Furthermore, FM1-43 uptake and 3H-gentamicin accumulation were decreased in hair cells in cultured organs of Corti isolated from varitint-waddler mice. Together, our data suggest a critical role of Mcoln3 at the ankle link region during the growth of the stereociliary hair bundle and indicate an inhibitory adverse effect of mutant Mcoln3 <I326T/A419P> on hair cell mechanotransduction. [unreadable] [unreadable] In order to understand the genetics and mechanistics of polygenic, non-mendelian inheritance of hearing loss, we surveyed outbred strains for hearing function. Outbred strains represent a genetic spectrum different from the common inbred strains and show a greater degree of allelic heterogeneity. They are models to isolate genetic interactions and to identify new deafness alleles. We found that the BlackSwiss outbred strains undergoes early-onset slow progressing hearing loss. The segregation pattern in intercross and backcross was consistent with a polygenic inheritance. Genome-wide linkage analyses on backcross and intercross populations identified and localized three quantitative trait loce (QTLs) underlying hearing loss in Black Swiss mice. A major QTL localized to chromosome 10 (named ahl5) and a second small-effect QTL localized to chromosome 18 (ahl6). Furthermore, four outbred strains with distinct hearing loss profiles were characterized. [unreadable] [unreadable] In a recent study, we refined the phenotypic and genetic parameters of the original jc mutation and characterized a new mutant allele, jc2J. In open-field behavior tests, homozygous jc mutants exhibited abnormal circling and ambulatory behavior that was indistinguishable from that of phenotypically similar mutants with defects in the vestibule of the inner ear. The jc/jc and jc2J/jc2J mice had stable elevated auditory-evoked brainstem response (ABR) thresholds at the 16 kHz stimulus of 88 dB sound pressure levels (SPL) and 43 dB SPL, respectively. Peak latencies and peak time intervals were normal in jc mutants. The jc mice showed no measurable distortion-product-otoacoustic-emissions (DPOAEs) above the system noise floor. In the mutant cochlea, the apical turn failed to form due to the developmental growth arrest of the cochlear duct at the level of the first turn at gestational day 13.5. In jc mutants, the cellular pattern of the organ of Corti is severely disrupted exhibiting supernumerary hair cells at the apex, mirror-image duplications of tunnel of Corti and inner hair cells, as well as ectopic formation of hair cells within Kllikers organ. These defects are caused by mutations in the novel Jxc1 gene, which encodes a nuclear protein with motif similarities to the polycomb group of transcriptional repressor that is expressed in developing sensory epithelia and ganglia. Collectively, our data support a role for Jxc1 in controlling a critical step of cochlea extension and cell fate.